1. Field of the Invention
The present invention relates to a projection exposure apparatus used in the manufacture of semiconductor integrated circuits or liquid crystal displays.
2. Related Background Art
In a conventional projection exposure apparatus, an attempt has been made to change the incident angle range (so-called .sigma. value=(numerical aperture of illumination light)/(numerical aperture of projection optical system)) of illumination light on a reticle pattern so as to increase the resolving power. A change in .sigma. value is generally realized by arranging a light-shielding member (stop) near a Fourier transform plane for a reticle pattern in an illumination optical system. A method (annular illumination method) of shielding or reducing a central portion of illumination light for setting an incident angle range on a Fourier transform plane can also be realized by adding a light-shielding member (light-reducing member).
A phase-shifting method has recently been proposed, and a phase shift reticle for realizing this method has become popular. For this reason, an illumination system having a small .sigma. value suitable for a phase-shifting method is required in a projection exposure apparatus. The annular illumination method has also become popular, and an appropriate annular illumination system is required. In either method, a higher resolving power and a larger focal depth can be obtained than those of a conventional exposure method. However, in the change method using the conventional light-shielding member as described above, a loss in illumination light amount upon partial light-shielding of the illumination light using the light-shielding member becomes large. The illumination intensity on a photosensitive substrate (wafer) is decreased to undesirably prolong the exposure time. The processing capacity per unit time in the projection exposure apparatus is decreased, resulting in a practical problem.